Process and apparatus for producing oxygen

Abstract: A process and apparatus for separating air in which compressed, purified air is cooled in a heat exchanger 18 and separated in a single rectification column 24 into nitrogen vapour at its top and liquid oxygen at its bottom. A nitrogen stream is withdrawn from the column 24 and is partly condensed in a condenser 32. Fluid from the condenser 32 is passed into a phase separator 36. A stream of the resulting vapour phase is warmed in the heat exchanger 18, and is expanded in a turboexpander 48. The resulting expanded vapour is passed through the heat exchanger 18 from its cold end to its warm end in order to provide refrigeration for the process. A stream of liquid oxygen is withdrawn from the bottom of the column 24, is reduced in pressure by passage through a valve 43, is vaporised by heat exchange with condensing nitrogen in the condenser 32 and is compressed cold in the compressor 54. One part of the compressed oxygen is taken as product, and another part recycled to the column 24. The turboexpander 48 is coupled to and drives the compressor 54.

Description: The present invention relates to a process and apparatus for rectifying air in a single column to produce oxygen. More particularly, the present invention relates to such a process and apparatus in which the single column operates at an above-atmospheric pressure to produce the oxygen at an above-atmospheric delivery pressure. The prior art has provided a variety of processes and apparatus to rectify air within various single column arrangements to produce an oxygen product. In a typical single column oxygen producing plant, air is compressed, purified, cooled to a temperature suitable for its rectification and then introduced into a heat exchanger in the bottom of the column to provide boil-up against the partial liquefaction of the air. The air is thereafter introduced into the column, at an intermediate location thereof. The air is distilled in the column to produce a liquid oxygen column bottom and a nitrogen vapour tower overhead. The column typically operates slightly above atmospheric pressure. As a result, the liquid oxygen must again be pumped to increase its pressure to a delivery pressure. As can be appreciated, such pumping represents an energy outlay which adds to the operating overhead involved in producing the oxygen product. As will be discussed, the present invention provides a process and apparatus in which air is distilled in a column to produce an oxygen product at an above-atmospheric delivery pressure without the necessity of there being any additional energy outlay involved in increasing the pressure of the oxygen product to the delivery pressure. US-A-4 966 002 relates to process and apparatus for producing nitrogen from air. A stream of air is compressed and purified. The resulting stream is cooled to a temperature suitable for its rectification in a main heat exchanger. The resulting cooled air is separated in a rectification column operating at a superatmospheric pressure into nitrogen vapour at its top and oxygen-rich liquid at its bottom. A stream of the oxygen-rich liquid is vaporised in indirect heat exchange with a stream of nitrogen taken from the top of the rectification column. The condensate is returned to the top of the column as reflux. Another stream of nitrogen vapour flows from the top of the column through the main heat exchanger from its cold end to its warm end and is taken as product. The vaporised oxygen-rich gas is divided into two parts. One part is "cold compressed" and is returned to the rectification column. The other part is partially warmed in the main heat exchanger and is expanded in an expander. Downstream thereof the expanded oxygen-rich fluid flows through the main heat exchanger from its cold end to its warm end and is thereby fully warmed. The compressor is coupled to the expander, typically through a dissipitive brake. GB-A-1 523 434 also relates to a process and apparatus for producing nitrogen. Air is separated into oxygen-rich liquid and nitrogen vapour fractions in a rectification column. Oxygen-rich liquid from the bottom of the column is employed to condense some of the top vapour. The oxygen-rich liquid is thereby vaporised. The resulting vapour is expanded in an expansion turbine which is coupled to a compressor that raises the pressure of the nitrogen product. According to the present invention there is provided a process of separating oxygen from air to form an oxygen product, said process comprising: compressing and purifying the air; cooling the air to a temperature suitable for its rectification; separating the air in a rectification column operating at a superatmospheric pressure into nitrogen vapour at its top and liquid oxygen at its bottom; removing from the column a refrigerant stream comprising nitrogen, a reflux stream composed of the top nitrogen vapour, and an oxygen stream composed of the said liquid oxygen; expanding the oxygen stream, vaporizing the expanded oxygen stream against the reflux stream, at least part of the reflux stream being condensed thereby, returning at least part of the reflux stream to the column as reflux, compressing the vaporised oxygen stream to at least the superatmospheric pressure of the column, and dividing the resulting compressed oxygen stream into two partial streams; cooling one of the partial streams and introducing the cooled partial stream into the bottom region of the column; partially warming the refrigerant stream against the air being cooled and the said partial stream being cooled, expanding the refrigerant stream with the performance of work, and, fully warming the expanded refrigerant stream against air being cooled and the partial stream being cooled; recovering the oxygen product from the other partial stream, wherein the said work comprises all that required to compress the vaporised oxygen stream. The invention also provides an apparatus for separating oxygen from air to produce an oxygen product, said apparatus comprising: means for compressing the air; means for purifying the air; heat exchange means for cooling the air to a temperature suitable for its rectification; a rectification column for separating the cooled into nitrogen vapour at its top and liquid oxygen at its bottom; means for condensing at least part of a reflux stream composed of the top nitrogen vapour against an expanded vaporising oxygen stream composed of the said liquid oxygen; means for returning at least part of the condensed reflux stream to the column; a recycle compressor communicating with the condenser means for compressing the oxygen stream to at least the operating pressure of the column; means communicating with the recycle compressor for dividing the compressed oxygen stream into two partial streams, said dividing means communicating with an inlet for one partial stream to the bottom of the column via the cold end of the heat exchange means, and with an outlet from the warm end of the heat exchanger means for a product oxygen stream comprising the other partial stream; means for taking a refrigerant stream comprising nitrogen from the column and passing it into the cold end of the heat exchange means; means for expanding the refrigerant stream with the performance of work, the expansion means having an inlet for partially warmed refrigerant communicating with an intermediate region of the heat exchange means and an outlet for expanded refrigerant communicating with a passage through the heat exchange means having an inlet at the cold end of the heat exchange means and an outlet at the warm end of the heat exchange means; wherein the expansion means is coupled to the recycle compressor such that all the work of compressing the oxygen stream is able to be provided by the expansion of the refrigerant stream. As can be appreciated, in any process and apparatus in accordance with the present invention, part of the work of expansion can be used to drive a recycle compressor used in compressing the oxygen to the delivery pressure. Since a partial stream from the recycle compressor is recovered as product, less energy need be expended than in prior art processes in raising the pressure of the product stream to the above-atmospheric delivery pressure. The invention will now be described by way of example with reference to the accompanying drawing which is a schematic flow diagram of an apparatus for performing a method in accordance with the present invention. It is understood that reference numerals designating process streams also designate piping used in connecting major components of the apparatus. With reference to the drawing, an apparatus 10 in accordance with the present invention is illustrated. In a conventional manner, air is compressed in an air compressor 12 to essentially the above-atmospheric delivery pressure. The heat of compression is removed by an aftercooler 14 and the compressed air is purified by a prepurification unit 16 (preferably a pressure swing adsorption (PSA) unit having beds of activated alumina and molecular sieve material) to remove carbon dioxide, moisture, and possibly hydrocarbons. The purified air, as an air stream 17, is cooled in a main heat exchanger 18 to a temperature suitable for rectification which would lie at or near the dew point of the air. The main heat exchanger 18 is preferably of plate-fin design. The cooled air is introduced as a stream 20 into a rectification column 24 having approximately 30 theoretical stages formed by trays of conventional design and efficiency, or the equivalent in structured or random packing or any other gas-liquid mass transfer element that could be used to bring into intimate contact ascending vapour and descending liquid phases within column 24. Column 24 has top and bottom regions 26 and 28 in which nitrogen vapour and liquid oxygen fractions are produced, respectively. The nitrogen vapour is removed from top region 26 of column 24 as a nitrogen reflux stream 30. Nitrogen reflux stream 30 is partially condensed within head condenser unit 32. Partially condensed reflux stream 34 is introduced into phase separator 36 to produce liquid and vapour phases. The liquid phase is returned to top region 26 of column 24 as reflux by way of reflux stream 38. The condensation within head condenser 32 is effected by withdrawing from the bottom region 28 of the column 24 an oxygen stream 40 composed of liquid oxygen. Oxygen stream 40 is sub-cooled within a sub-cooler 42 and the sub-cooled oxygen is lowered in temperature by irreversible expansion within a pressure reduction valve 43 upstream of its being introduced into head condenser 32. The sub-cooler 42 is of conventional plate-fin design. It is understood that an embodiment of the present invention is possible in which nitrogen reflux stream 30 is fully condensed and all or some of the condensate is returned to top region 26 of column 24. That part of the condensate not returned could be routed through sub-cooler 42 counter-current to the direction of flow of oxygen stream 40 and then through main heat exchanger 18 in a direction counter-current to the air feed. Refrigeration is supplied in order to balance heat leakage into the cold box and the warm end heat losses. To this end, the vapour phase produced within phase separator 36 is withdrawn as a nitrogen stream 44 which is sent through sub-cooler 42 in order to help sub-cool oxygen stream 40. Stream 44 is sent through the main heat exchanger which is provided with a first passage 45 through which air passes from purification unit 16 into column 24. The main heat exchanger is also provided with a second passageway 46 in which the nitrogen stream partially warms by passing in a direction countercurrently to the flow of air. In this regard, the term "fully warmed" means that a stream has been warmed to the ambient, that is, the warm end of the main heat exchanger, "fully cooled" means the stream has been cooled to a temperature of the cold end of the main heat exchanger, namely at about the dew point of air. "Partially cooled" or "partially warmed" means that the stream either passes in a direction of the air flow or counter-currently to the direction of the air flow, respectively, and is withdrawn from the main heat exchanger at a temperature intermediate that of the warm and cold ends of the main heat exchanger. Downstream of its having been partially warmed, nitrogen stream 44 is introduced into a turboexpander 48 or other machine capable of expanding stream 44 with the performance of work to produce a refrigerant stream 50. Refrigerant stream 50 passes in sequence through subcooler 42 where it aids in subcooling oxygen stream 40 and through a third passageway 52 of the main heat exchanger in which it fully warms and passes out of apparatus 10 as a waste stream or possibly as a low pressure nitrogen co-product. Refrigerant stream 50 passes through a third passage of the main heat exchanger 18, in a counter-current direction to the entering air flowing through the first passageway 45. The enthalpy of the incoming air is thereby lowered to add refrigeration to the system. It is to be noted in an alternative embodiment of the present invention, the refrigerant stream could be formed from nitrogen-rich vapour taken from a liquid-vapour contact level beneath the uppermost such level in the column 24. In such case, all or a portion of the nitrogen tower vapour overhead would be used as reflux. An oxygen vapour stream 56 passes from the condenser 32 into a recycle compressor 54 where it is compressed to a pressure sufficiently above that at the bottom region 28 of the column 24 to enable a stream of the compressed oxygen to be introduced into the bottom region 28. Compressor 54 is driven by turboexpander 48 through a heat dissipative brake 60 which rejects excess work of expansion from the cold box (not shown) as heat. Oxygen stream 56 is therefore compressed cold at, column temperature. This is preferred to compressing oxygen which has been fully or partially warmed because of reduced work requirements involved in compressing cold oxygen. Compressed oxygen stream 58 flows from the compressor 54 and is divided into two partial streams 62 and 64 either upstream of or within main heat exchanger 18. Partial stream 62 is cooled to a temperature near its dew point in a fourth passage 66 of the main heat exchanger 18. The cooled partial oxygen stream is introduced as essentially a vapour into bottom region 28 of column 24 to provide boil-up in such bottom region. It is to be noted that the term "essentially" here connotes that there can be some liquid content, for instance in the neighbourhood of 2%. The other of the partial streams 64 is fully warmed within main heat exchanger 18 by flow through a fifth passage 68 thereof. After being fully warmed, the stream is taken off as the oxygen product. Partial stream 64 could be removed as a product without passing it through main heat exchanger 18. In such case, recovery would be reduced. EXAMPLE The following is a computer simulation of a typical operation of apparatus 10.

Claims: A process of separating oxygen from air to form an oxygen product (64), said process comprising: compressing and purifying the air; cooling the air to a temperature suitable for its rectification; separating the air in a rectification column (24) operating at a superatmospheric pressure into nitrogen vapour at its top (26) and liquid oxygen at its bottom (28); removing from the column (24) a refrigerant stream (44, 46) comprising nitrogen, a reflux stream (38) composed of the top nitrogen vapour, and an oxygen stream (40, 56) composed of the said liquid oxygen; expanding the oxygen stream, vaporizing the expanded oxygen stream (40, 56) against the reflux stream (38), at least part of the reflux stream (38) being condensed thereby, returning at least part of the reflux stream to the column (24) as reflux, compressing the vaporised oxygen stream (40, 56) to at least the superatmospheric pressure of the column (24), and dividing the resulting compressed oxygen stream (58) into two partial streams (66, 68); cooling one (66) of the partial streams (66, 68) and introducing the cooled partial stream (66) into the bottom region (68) of the column (24); partially warming the refrigerant stream (44, 46) against the air being cooled and the said partial stream (66) being cooled, expanding the refrigerant stream (44, 46) with the performance of work, and, fully warming the expanded refrigerant stream (50) against air being cooled and the partial stream (66) being cooled; recovering the oxygen product (64) from the other partial stream (68), wherein the said work comprises all that required to compress the vaporised oxygen stream (40, 56). A process as claimed in claim 1, wherein the oxygen stream (40, 56) is compressed at the column (24) temperature. A process as claimed in claim 1 or claim 2, in which the air is introduced into the column at an intermediate liquid-vapour contact level thereof. An apparatus for performing a process according to any one of the preceding claims, said apparatus comprising: means (12) for compressing the air; means (16) for purifying the air; heat exchange means (18) for cooling the air to a temperature suitable for its rectification; a rectification column (24) for separating the cooled into nitrogen vapour at its top (26) and liquid oxygen at its bottom (28); means (32) for condensing at least part of a reflux stream (38) composed of the top nitrogen vapour against an expanded vaporising oxygen stream (40, 56) composed of the said liquid oxygen; means (36) for returning at least part of the condensed reflux stream (38) to the column (24); a recycle compressor (54) communicating with the condenser means (32) for compressing the oxygen stream (40, 56) to at least the operating pressure of the column; means communicating with the recycle compressor (54) for dividing the compressed oxygen stream (58) into two partial streams (66, 68), said dividing means communicating with an inlet for one partial stream (66) to the bottom of the column (24) via the cold end of the heat exchange means (18), and with an outlet from the warm end of the heat exchange means (18) for a product oxygen stream (64) comprising the other partial stream (68); means for taking a refrigerant stream (44, 46) comprising nitrogen from the column (24) and passing it into the cold end of the heat exchange means (18); means (48) for expanding the refrigerant stream (44, 46) with the performance of work, the expansion means (48) having an inlet for partially warmed refrigerant communicating with an intermediate region of the heat exchange means (18) and an outlet for expanded refrigerant communicating with a passage through the heat exchange means (18) having an inlet at the cold end of the heat exchange means (18) and an outlet at the warm end of the heat exchange means (18); wherein the expansion means (48) is coupled to the recycle compressor (54) such that all the work of compressing the oxygen stream (40, 56) is able to be provided by the expansion of the refrigerant stream (44, 46). Apparatus according to claim 4, wherein: the expansion means (48) comprises a turboexpander (48); and the turboexpander (48) is connected to the recycle compressor (54) by an energy dissipative brake. Apparatus according to claim 4 or claim 5, wherein: the reflux return means (36) comprises a phase separation tank (36) having an inlet communicating with the condenser means (32), an outlet for liquid communicating with the top (26) of the column (24), and an outlet for vapour communicating with an inlet to the expansion means (48). Apparatus according to any one of claims 4 to 6, wherein: the recycle compressor (54) so communicates with the condenser means (36) that it receives, in use, the oxygen at essentially the operating temperature of the column (24). Apparatus according to any one of claims 4 to 7, wherein there is an inlet for the cooled air at an intermediate liquid-vapour contact level of the column (24).

Inventors: NAUMOVITZ JOSEPH P, NAUMOVITZ, JOSEPH P.
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1994
Main classification: F25J3
IPC codes: F25J3